Medium large transport flight crew has loss of pressurization at FL370. On descent they were able to maintain pressure and returned to FL330.

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Attributes
ACN	329847
Time
Date	199603
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : pxv
State Reference	IN
Altitude	msl bound lower : 19000 msl bound upper : 37000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid tower : vgt
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 17000 flight time type : 800
ASRS Report	329847
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : declared emergency
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

At cruise FL370 the cabin altitude warning horn came on. Cabin altitude was 10000 ft and climbing 300 FPM. We ran the loss of pressurization checklist and tried to regain control of the cabin. The outflow valve was closed, duct pressure normal, 2 packs and 2 bleeds operating normally. We could see no reason for the problem. We turned the packs on high. As we descended we observed the cabin again begin to pressurize. We requested a level off with ATC at 19000 ft and allowed the cabin to stabilize and asked for a climb back to altitude of 33000 ft. The system operated normally in all modes. After landing in cleveland and talking to maintenance we discovered the forward cargo door slightly ajar and a line of ice between the door and fuselage. I suspect there was a problem with the door not closing all the way but enough where we didn't get a light (warning). I believe the pressure differential at FL370 was enough to push the door (which was slightly sprung or not closed tight) out enough to cause a slow leak. It sealed ok at FL330 and below. This is my theory as I was unable to follow up with maintenance.

Original NASA ASRS Text

Title: MLG FLC HAS LOSS OF PRESSURIZATION AT FL370. ON DSCNT THEY WERE ABLE TO MAINTAIN PRESSURE AND RETURNED TO FL330.

Narrative: AT CRUISE FL370 THE CABIN ALT WARNING HORN CAME ON. CABIN ALT WAS 10000 FT AND CLBING 300 FPM. WE RAN THE LOSS OF PRESSURIZATION CHKLIST AND TRIED TO REGAIN CTL OF THE CABIN. THE OUTFLOW VALVE WAS CLOSED, DUCT PRESSURE NORMAL, 2 PACKS AND 2 BLEEDS OPERATING NORMALLY. WE COULD SEE NO REASON FOR THE PROB. WE TURNED THE PACKS ON HIGH. AS WE DSNDED WE OBSERVED THE CABIN AGAIN BEGIN TO PRESSURIZE. WE REQUESTED A LEVEL OFF WITH ATC AT 19000 FT AND ALLOWED THE CABIN TO STABILIZE AND ASKED FOR A CLB BACK TO ALT OF 33000 FT. THE SYS OPERATED NORMALLY IN ALL MODES. AFTER LNDG IN CLEVELAND AND TALKING TO MAINT WE DISCOVERED THE FORWARD CARGO DOOR SLIGHTLY AJAR AND A LINE OF ICE BTWN THE DOOR AND FUSELAGE. I SUSPECT THERE WAS A PROB WITH THE DOOR NOT CLOSING ALL THE WAY BUT ENOUGH WHERE WE DIDN'T GET A LIGHT (WARNING). I BELIEVE THE PRESSURE DIFFERENTIAL AT FL370 WAS ENOUGH TO PUSH THE DOOR (WHICH WAS SLIGHTLY SPRUNG OR NOT CLOSED TIGHT) OUT ENOUGH TO CAUSE A SLOW LEAK. IT SEALED OK AT FL330 AND BELOW. THIS IS MY THEORY AS I WAS UNABLE TO FOLLOW UP WITH MAINT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.